Source : Free On-Line Dictionary of Computing

Magic Switch Story
     
        Some years ago, I was snooping around in the cabinets that
        housed the {MIT AI Lab}'s {PDP-10}, and noticed a little
        switch glued to the frame of one cabinet.  It was obviously a
        homebrew job, added by one of the lab's hardware hackers
        (no-one knows who).
     
        You don't touch an unknown switch on a computer without
        knowing what it does, because you might crash the computer.
        The switch was labelled in a most unhelpful way.  It had two
        positions, and scrawled in pencil on the metal switch body
        were the words "magic" and "more magic".  The switch was in
        the "more magic" position.
     
        I called another hacker over to look at it.  He had never seen
        the switch before either.  Closer examination revealed that
        the switch had only one wire running to it!  The other end of
        the wire did disappear into the maze of wires inside the
        computer, but it's a basic fact of electricity that a switch
        can't do anything unless there are two wires connected to it.
        This switch had a wire connected on one side and no wire on
        its other side.
     
        It was clear that this switch was someone's idea of a silly
        joke.  Convinced by our reasoning that the switch was
        inoperative, we flipped it.  The computer instantly crashed.
     
        Imagine our utter astonishment.  We wrote it off as
        coincidence, but nevertheless restored the switch to the "more
        magic" position before reviving the computer.
     
        A year later, I told this story to yet another hacker, {David
        Moon} as I recall.  He clearly doubted my sanity, or suspected
        me of a supernatural belief in the power of this switch, or
        perhaps thought I was fooling him with a bogus saga.  To prove
        it to him, I showed him the very switch, still glued to the
        cabinet frame with only one wire connected to it, still in the
        "more magic" position.  We scrutinized the switch and its lone
        connection, and found that the other end of the wire, though
        connected to the computer wiring, was connected to a ground
        pin.  That clearly made the switch doubly useless: not only
        was it electrically nonoperative, but it was connected to a
        place that couldn't affect anything anyway.  So we flipped the
        switch.
     
        The computer promptly crashed.
     
        This time we ran for Richard Greenblatt, a long-time {MIT}
        hacker, who was close at hand.  He had never noticed the
        switch before, either.  He inspected it, concluded it was
        useless, got some diagonal cutters and {dike}d it out.  We
        then revived the computer and it has run fine ever since.
     
        We still don't know how the switch crashed the machine.  There
        is a theory that some circuit near the ground pin was
        marginal, and flipping the switch changed the electrical
        capacitance enough to upset the circuit as
        millionth-of-a-second pulses went through it.  But we'll never
        know for sure; all we can really say is that the switch was
        {magic}.
     
        I still have that switch in my basement.  Maybe I'm silly, but
        I usually keep it set on "more magic".
     
        {GLS}
     
        (1995-02-22)